1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method for the same, and more particularly to a semiconductor device comprising an N-type MOS transistor and a P-type MOS transistor, which utilize conductor films in the gate electrodes, and to a manufacturing method for the same.
2. Description of the Related Art
Device downscaling has been pursued in order to achieve a higher performance MOSFET (Metal Oxide Semiconductor Field Effect Transistor), and also the need to reduce power consumption has been increased in addition to the device downscaling. In order to reduce power consumption, it is necessary to keep the threshold value of a transistor at a low value. Therefore, gate electrodes having different work functions have been used for N-type MOSFET and P-type MOSFET, respectively.
Generally speaking, polycrystalline silicon (polysilicon) is utilized in the gate electrode of a transistor, and a low threshold value is achieved by doping impurities into the polysilicons, which are the gate electrode of N-type MOSFET and P-type MOSFET, converting these polysilicons to n-type polysilicon and p-type polysilicon, respectively, and setting the work function of the respective polysilicons in the proximity of the conduction band and valence band.
However, in a gate electrode comprised of polysilicon, even if doping is done in high concentrations so that the impurity concentration is at the 1020 cm−3 level, which is the solid solubility limit of a conductive impurity, because a depletion layer is formed on the gate electrode side, gate capacitance decreases to that extent. Thus, when forming a gate insulation film, it is necessary to make it approximately 0.5 nm thinner in anticipation of depletion layer gate capacitance, but the current situation is such that it is difficult to make the gate insulation film thinner due to the problem of gate leakage current increase caused by a gate insulation film tunnel current.
As measures for avoiding the problem, increasing the dielectric constant of the gate insulation film, and the utilization of metal gate electrodes are being studied. Increasing the dielectric constant of the gate insulation film achieves the physical thickness of the gate insulation film, and holds down tunnel current by replacing the gate insulation film with a high dielectric layer. Recently, the development of materials for high dielectric gate insulation films has been vigorously pursued, but these materials have yet to reach the point where they can be considered as much reliable as the conventional silicon oxide layer, and it will still be some time before they can be applied to actual devices.
The use of a metal gate electrode prevents the depletion of the gate electrode by replacing the polysilicon gate electrode by metal one. When a metal gate electrode is employed to maintain the threshold value of the transistor at a low value, a device is formed by using a metal having a work function in the vicinity of 4.0 eV, which is the conduction band of silicon, as the gate electrode material in an N-type MOSFET, and using a metal having a work function in the vicinity of 5.1 eV, which is the valence band of silicon, as the gate electrode material in a P-type MOSFET (for example, refer to Japanese Laid-open Patent No. 2000-31296 (corresponding U.S. Pat. No. 6,027,961) and Japanese Laid-open Patent No. 2000-252371 (corresponding U.S. Pat. No. 6,291,282)).
However, in a conventional device, which uses polysilicon for the gate electrode, the formation of the gate electrodes of the N-type MOSFET and the P-type MOSFET is carried out simultaneously, but in manufacturing the device disclosed in Japanese Laid-open Patent No. 2000-31296 (corresponding U.S. Pat. No. 6,027,961), the problem was that the formation of the respective gate electrodes was carried out separately, thereby greatly increasing the number of processes.
With the forgoing in view, it is an object of the present invention to provide a semiconductor device and manufacturing method therefor, which makes possible the formation of metal gate electrodes having different work functions in a P-type MOSFET and an N-type MOSFET without significantly increasing the number of manufacturing processes.